FIG. 9 illustrates the most commonly adopted electrode plate structure for coin type batteries, in which a disk-shaped positive electrode pellet 32 and a negative electrode pellet 33 are arranged face-to-face with a separator 34 interposed in between them. This coin type battery is manufactured through the following process steps to have the coin-like appearance: The positive electrode pellet 32, separator 34, and negative electrode pellet 33 are placed in the face-to-face arrangement inside a sealing case 35 of circular half shell form, liquid electrolyte is injected, a gasket 36 is fitted to a lateral periphery of the sealing case 35, a cap case 31 is coupled onto the sealing case, and the open end edge of the cap case 31 is crimped or bent inwards so as to tightly seal the battery inner space.
Batteries having such electrode plate structure as described above wherein one positive electrode pellet 32 and one negative electrode pellet 33 are arranged face-to-face with each other cannot have a large discharge capacity because of the small reaction area where the positive and negative electrode plates face each other. To achieve a larger discharge capacity, positive and negative electrode plates must face each other in a larger area, and accordingly, for relatively larger batteries other than coin type batteries, various structures that enlarge the reaction area have widely been adopted in an attempt to increase the battery energy density in high rate discharge performance. There is, for example, a layered structure in which a plurality of positive and negative electrode plates are layered upon one another with separators interposed therebetween. Another example is a coiled structure in which strips of positive and negative electrode plates are coiled together with a separator interposed therebetween. The discharge capacity of coin type battery could be increased if electrode plates of such a layered or coiled structure were accommodated in a coin-shaped, flat case. In fact, the applicants of the present invention have proposed a battery containing an electrode plate group of a coiled structure in a coin-shaped, flat case in Japanese Patent Laid-Open Applications Nos. 2000-241678 and 2000-241679.
This electrode plate group of the coiled structure is constructed as shown in FIG. 10A and FIG. 10B: A positive electrode plate 7 consists of a plurality of positive electrode layer faces 17a-17e coupled together by connecting pieces 19a-19d, and a negative electrode plate 8 consists of a plurality of negative electrode layer faces 18a-18e that are larger than the positive electrode layer faces 17a-17e and coupled together by connecting pieces 20a-20d. They are coiled around with a separator interposed between them into a flat shape so as to be accommodated in a circular case. The resultant flat shape electrode plate group 1 appears as shown in FIG. 11. As shown in FIG. 12, this electrode plate group 1 is encased in a sealing case 5, a negative electrode lead 16 is welded to an inner face of the sealing case 5, and a positive electrode lead 15 is welded to a cap case 4. The sealing case 5 is then filled with liquid electrolyte. A gasket 6 is fitted to a lateral periphery of the sealing case 5, the cap case 4 is coupled thereon, and the open end edge of the cap case 4 is crimped so as to tightly seal the battery inner space. Such coin type battery can be constructed both as a lithium primary battery and rechargeable battery. Such a small and flat shape battery capable of outputting a large discharge current can contribute to realization of even smaller portable electronic devices and the like with higher functionalities.
One problem is that, in the manufacture of the electrode plate group 1, the positive and negative electrode plates 7, 8 are not fixed in position relative to each other, and they can easily be displaced in the process step of coiling them into a flat shape. If the positive electrode layer faces 17a-17e upon the negative electrode layer faces 18a-18e are largely offset because of the displacement during the coiling process, there is a risk of internal short-circuiting caused by dendritic growth of lithium. Since the coin type battery contains a coiled electrode plate group 1 in a small, flat case, internal short-circuiting may also be caused if the electrode plates are not in register, as they may contact the sealing case 5 upon an impact of a falling accident or the like.
Accordingly, it is necessary to detect the presence of discrepancy in position between the positive electrode layer faces 17a-17e and negative electrode layer faces 18a-18e, which is, however, not possible after the positive and negative electrode plates 7, 8 have been coiled around with the separator therebetween, because the finished electrode plate group 1 is covered by the separator that is unified with the electrode plates. Thus there is a need to inspect, by some means, whether there is a discrepancy in position between the positive electrode layer faces 17a-17e and negative electrode layer faces 18a-18e, so that only the electrode plate groups 1 that are within a permissible range of positional displacement are used for assembling the batteries.
An object of the present invention is to provide a coin type battery having a feature that enables detection of positional displacement between positive and negative electrode plates of an assembled electrode plate group, and a manufacturing method of this coin type battery.